1. Field of the Invention
The present invention relates generally to acoustic sensing systems, and more specifically relates to a system for sensing acoustic waves comprising an acoustic sensor array.
2. Description of the Related Art
Typically, to obtain oil, a well or hole is dug by drilling and removing earth from the ground to form a shaft known as a xe2x80x9cborehole,xe2x80x9d which extends to the bottom of the well. Generally, a large metal pipe or casing will be inserted into the borehole. Smaller pipes, known as production tubes, are inserted into the casing. These production tubes allow access to the bottom of the well. For example, oil may be drawn from the well through the production tubing.
Ultimately, the well will appear to go dry. Despite the apparent lack of oil within the well, vast supplies of oil are often trapped in pockets in the earth nearby the well. These pockets, however, are generally inaccessible to the drilled well. To locate such pockets, known in the art as xe2x80x9cin-placexe2x80x9d reserves, geologists conduct surveys of swaths of earth surrounding the wells. Geologists employ techniques like cross-well tomography in which acoustic waves are transmitted through a volume of earth to characterize properties, such as density, in that volume. Knowledge of the density of the earth helps determine the presence or absence of oil in the region of the earth being characterized.
To survey the transmission characteristics of a region of the earth, an acoustic wave source can be used to generate acoustic waves, i.e., sound, while an array of acoustic sensors detects these acoustic waves. Generally, each of the sensors in the array will be situated at a different location. The acoustic waves emitted from the acoustic source are thus sampled at a plurality of pints which typically make up a line. By changing the location of the acoustic source, the location of the sensor array, or both, the transmission characteristics of a volume of earth may be measured. In this manner, a three-dimensional map of the density throughout a region of earth can be produced.
Although some prior art techniques rely on acoustic sources and/or sensor arrays situated on the surface of the earth, placing the acoustic sources and sensor arrays deep within the earth is more effective for surveying lower regions of the earth. To conduct measurements deep within the earth, a probe can be lowered into the well.
However, the frailty of conventional prior art sensors prevents prior art sensor arrays from being employed deep within a well. Conventional sensor arrays employ piezoelectric transducers (or piezos) to convert vibrations originating from the acoustic waves into electronic signals. Since a piezoelectric transducer outputs only a small signal, an electronic preamplifier must be mounted near the piezo to prevent noise from overwhelming the small transducer signal. Electronics, however, are incompatible with the harsh environmental conditions, such as high temperature and pressure, that prevail deep within the earth. Even preamplifiers designed to survive high temperature have a short lifetime and may last, for example, only for one hour under harsh conditions. Thus, the requirement for an electronic preamplifier prevents piezoelectric transducers from being employed deep within a well.
Fiber optic sensors, on the other hand, are electrically passive devices. That is, they do not require electrical components or external electrical connections. Thus they are less susceptible to the harshness associated with high temperature, high pressure environments. Furthermore, fiber optic sensors avoid the environmental problems associated with electrical components, e.g., the electromagnetic interference that arises when electrical components are placed in the presence of transmission lines. For these reasons, fiber optic sensors are sometimes used in hydrophones operating under harsh environmental conditions.
Fiber optic hydrophones can generally be classified into two categories. Hydrophones of the air backed mandrel design have a hollow, sealed cavity that deforms in response to acoustic pressure, so that strain is transferred to the fiber wrapped around the mandrel. Other, less sensitive, fiber optic hydrophone designs record the effects of pressure directly on the fiber itself, e.g., the fiber may be wrapped around a solid body. Fiber optic hydrophones with high sensitivity (i.e., air backed mandrel hydrophones) are generally limited to operating pressures of less than about 5000 pounds per square inch (psi) and temperatures of less than about 120xc2x0 C. Outside this range, the materials used in the mandrels of air backed mandrel hydrophones deform excessively. For example, polycarbonate plastic deforms at these temperatures, whereas metals such as aluminum buckle inelastically when subjected to high pressures. On the other hand, fiber optic hydrophones utilizing solid bodies or fiber for acoustic transduction typically have much lower sensitivities.
In addition to operating limitations on pressure and temperature, current fiber optic hydrophones are generally bulky, and may have large cross sections that do not lend themselves to use in applications where compactness is essential, e.g., in commercial petrochemical wells and boreholes. Thus, there is a need for a fiber optic hydrophone having a relatively small cross section and the ability to withstand high pressures and temperatures.
In addition to restrictions on the placement of the prior art acoustic arrays, limitations exist on the number of sensors that may be employed in prior art acoustic arrays. With a larger number of sensors more information must be processed. Limitations on the amount of information that can be processed within a reasonable amount of time restrict the number of sensors that can be used. Higher resolution maps, however, can be achieved with a larger number of sensors.
Thus, a need exists for a system for sensing acoustic waves that is rugged enough to operate in the harsh downhole environment and accommodates a large number of sensors.
Systems accommodating a large number of sensors may benefit from the use of multiplexing, in which multiple signals are communicated within a single line. One common approach, known as frequency division multiplexing (FDM), operates by modulating a carrier wave at a number of different frequencies equal to the number of signals that are to be multiplexed. When FDM is applied to a system using interferometric sensors, the multiplexed signal includes signal components not just at the modulation frequencies, but at all harmonic frequencies of the modulation frequencies as well. For such a system, the multiplexed signal may be demultiplexed through detection of the signal components at the modulation and first harmonic frequencies, provided these components do not overlap (in frequency) one another or any components at the higher harmonics. Such overlap may be prevented by selecting modulation frequencies that are sufficiently large and separated that the lowest second order harmonic component exceeds the highest first harmonic component. This leads to large bands of unused frequency between DC and the highest frequency signal component detected. However, to keep the signal processing electronics simple it is preferable to keep the maximum frequency detected as low as possible. Thus, a need exists for a method of selecting a set of FDM modulation frequencies having as low a maximum frequency as possible while maintaining fundamental and first harmonic signal components that are not overlapped by other signal components.
The present invention comprises a system for sensing subterranean acoustic waves emitted from an acoustic source. The system comprises at least one optical source emitting light. A plurality of optical sensors receive the light and alter the light in response to the acoustic waves. At least one optical detector receives the altered light and outputs an electrical signal. The system also comprises electronics that receives the electrical signal and converts the signal into seismic acoustic data format.
The system advantageously comprises at least one distribution optical fiber line that distributes the light emitted from the optical source to the sensors as well as at least one return optical fiber line that directs the altered light emitted from the sensors to the at least one optical detector.
Preferably, the light emitted from the at least one optical source is modulated at at least one modulation frequency. The electronics advantageously demultiplex or demodulate the electrical signal by mixing the signal with periodic waveforms having frequencies corresponding to the modulation frequencies and twice the modulation frequencies.
This system may comprise a number of different types of fiber optic sensor inputs, specifically land seismic, downhole, and ocean bottom cables using either hydrophones, geophones, or a combination of both.
Another embodiment of the present invention comprises a downhole system for sensing acoustic waves emitted from a surface acoustic source or an underground acoustic source. The system comprises at least one optical source emitting light. An array of downhole optical sensors receives the light. This array alters the light in response to the acoustic waves. A plurality of optical detectors receives the altered light and outputs electrical signals. Electronics process these electrical signals, converting the signals into seismic data format.
In another embodiment, a downhole system for performing cross-well tomography comprises a plurality of laser sources each emitting light that is modulated at different frequencies. This downhole system further comprises an array of downhole optical sensors. These downhole optical sensors receive the light and alter it in response to a acoustic waves that impinge upon the acoustic sensors. A plurality of optical detectors receive the altered light and output electrical signals. Electronics process the electrical signals converting the signals into seismic data format.
One additional embodiment involves another system for sensing subterranean acoustic waves emitted from an acoustic source. This system comprises means for producing a plurality of coherent beams of light and a means for modulating the plurality of coherent beams of light at different frequencies. The system further comprises means for altering the beams of light in response to the acoustic waves. Variations in the beam of light are thereby produced. The system additionally comprises means for detecting the variations in the beam of light. Means for converting the detected variations into seismic data format are also included.
Another embodiment comprises a method for sensing subterranean acoustic waves emitted from an acoustic source. The method comprises producing at least one optical beam of light. The beam of light is altered in response to the acoustic waves. Variations in the beam of light are thereby produced. These variations in the beam of light are detected, and the detected variations are converted into seismic data format.
Another method for sensing subterranean acoustic waves emitted from an acoustic source comprises producing a plurality of coherent beams of light. The plurality of coherent beams of light are modulated at different frequencies. The beams of light are altered in response to the acoustic waves thereby producing variation in the beams of light. The altered beams of light are received, and electrical signals are output. The electrical signals are processed and converted into data in seismic data format.